1. Field of the Invention
The invention relates to the collective ejection of ions from three-dimensional Paul RF ion traps, particularly for their transfer to Kingdon ion traps or other mass analyzers.
2. Description of the Related Art
Mass spectrometers can only ever determine the ratio of the ion mass to the charge of the ion. In the following, the term “mass of an ion” or “ion mass” always refers to the ratio of the mass m to the dimensionless number z of excess positive or negative elementary charges of the ion, i.e. the elementary charge related mass m/z (or charge related mass, for short). There are various criteria for characterizing the performance of a mass spectrometer, the key one being the mass resolution. Mass resolution is usually defined as R=(m/z)/Δ(m/z)=m/Δm, where R is the resolution, m the mass of an ion, measured in units of the mass scale, and Δm the width of the mass signal at half maximum, measured in the same units. A special feature of radio-frequency ion traps (RF ion traps) is that R increases in proportion to m, which means that the signal width WIT=Δm, measured in units of the mass scale, remains constant with mass, and can simply regarded as a kind of mass resolution of the ion trap.
A three-dimensional Paul RF ion trap (3D RF ion trap) consists of two end cap electrodes and one ring electrode, as shown in FIG. 1. All electrodes usually have rotationally hyperbolic surfaces. High quality ion traps are operated with RF voltages of up to 30 kilovolts peak-to-peak and frequencies of around one megahertz. They form a quadrupole pseudopotential well inside the ion trap, with a quadratic increase of the pseudopotentials extending uniformly from the center of the ion trap in all three dimensional directions. In this pseudopotential well, the ions can oscillate harmonically through or about the center with a mass-dependent “secular frequency” (as if they were in a trough of real potentials, although this cannot be realized for all spatial directions simultaneously with electrostatic potentials). The ion traps are usually operated with a damping gas at a pressure of 0.1 to 1.0 pascal in order to damp (“cool”) the ion oscillations in the pseudopotential well of the ion trap, so the ions gather in the center. The damping process reduces the oscillation amplitudes exponentially with a time constant of a few tenths of a millisecond, so the ions come to rest in the form of a small cloud after around one to two milliseconds. The cloud has the shape of a flat ellipsoid of rotation. The diameters of the cloud in the axial and transverse directions result from the balance between space-charge forces acting centrifugally and forces of the pseudopotential acting centripetally. Under optimum operation conditions, the larger diameter in the transverse direction is of the order of a half to a full millimeter.
The ions can be ejected from the center through an opening in one of the end cap electrodes in ascending order of their mass-to-charge ratio m/z by means of special scan methods, usually by resonance excitation with an excitation high frequency voltage across the end cap electrodes. The ion current which leaves the trap can be measured with an ion detector as a function of time: this operation results in a mass spectrum. In high-quality ion trap mass analyzers, at scan speeds of 30,000 daltons per second, it is possible to achieve mass widths of around WIT=0.2 dalton in mass ranges up to 3000 daltons. At a mass of m=200 Da, this corresponds to a resolution of only R(200 Da)=1000; in higher mass ranges R(1000 Da)=5000 and R(3000 Da)=15,000 are achieved. These mass resolutions are only moderately good in the lower and medium mass range.
In radio-frequency ion traps, the ions can be manipulated in numerous ways. It is possible, for example, to “isolate” individual ion species by ejecting all other ion species by means of resonance or other processes. The isolated ions can be fragmented by collision-induced dissociation (CID) or by electron transfer (ETD) of suitable negative radical ions, for example. Electron transfer dissociation has the best yield of fragment ions in 3D RF ion traps, far better than in the linear two-dimensional RF ion traps (2D RF ion traps) which are usually used for this process. The 3D RF ion traps thus form a tandem mass spectrometer by themselves for the acquisition of fragment ion spectra (“tandem in time”). The isolated ions can also be subjected to quite different reactions; it is possible to reduce the charge number of multiply positively charged ions. These diverse possibilities make the 3D RF ion trap an excellent research tool for determining the structure of ions, their identity, their reactivity and many other properties. Moreover, 3D RF ion traps have a very high sensitivity, comparatively speaking, which opens up many fields of application.
These outstanding possibilities are accompanied by only a single disadvantage: the mass resolution and the associated mass accuracy are only moderately good. There is therefore a need for a device which combines the 3D RF ion trap, with its good ion manipulation possibilities, with a mass analyzer of maximum mass resolution. Ion cyclotron resonance mass spectrometers (ICR-MS), time-of-flight mass spectrometers (TOF-MS), and particularly Kingdon ion traps can be used as this type of mass analyzer.
Kingdon ion traps are generally electrostatic ion traps in which ions can orbit one or more inner electrodes or oscillate between several inner electrodes. An outer, enclosing housing is at a DC potential which the ions with a given total energy (sum of kinetic and potential energy) cannot reach. In special Kingdon ion traps, which are suitable as mass analyzers, the inner surfaces of the housing electrodes and the outer surfaces of the inner electrodes are designed so that, firstly, the motions of the ions in the longitudinal direction of the Kingdon ion trap are decoupled from their transverse motions as completely as possible and, secondly, a parabolic potential profile is generated in the longitudinal direction in which the ions can oscillate harmonically in the longitudinal direction. In this publication, the term “Kingdon ion trap” refers only to these special forms in which ions can oscillate harmonically in the longitudinal direction, decoupled from their transverse motions as far as possible. This harmonic oscillation is mass-dependent. If the image currents of these oscillations are measured at suitable electrodes, a Fourier analysis can be used to obtain a frequency spectrum, which is then converted into the mass spectrum. As with other Fourier transform mass spectrometers, for example ion cyclotron resonance mass spectrometers, a very high mass resolution R can be achieved for ions of the lower and medium mass range in particular. In Kingdon ion traps, a mass resolution of R(200 Da)=100,000 can easily be achieved for ions with a mass of m=200; for this, the measurement of the image current transient must have a specific duration. However, for the same measurement duration, mass resolution decreases toward higher masses in inverse proportion to the square root of the mass, so at m=3200 Da, a mass resolution of R(3200 Da)=25,000 is still achieved. Coupling a 3D Paul RF ion trap to a Kingdon ion trap therefore provides an ideal solution when good mass resolution for ions of lower masses is required.
Coupling 3D Paul R F ion traps to Kingdon ion traps has already been proposed in the document DE 10 2009 020 886 A1 (C. Köster and J. Franzen; corresponding to GB 2 470 259 A, US 2013/0146761 A1, US 2010/0301204 A1), but without stating how the different ion species can be ejected without this having a detrimental effect on the normal operation of a mass spectrometer based on the 3D RF ion trap.
The patent specification U.S. Pat. No. 5,886,346 A (A. A. Makarov) elucidates the fundamentals of a Kingdon ion trap which is marketed by Thermo-Fischer Scientific GmbH Bremen under the name Orbitrap®. The Orbitrap® has a spindle-shaped inner electrode within the coaxial housing electrode, which is split transversely in the center. An ion-optical device is used to inject the ions tangentially as ion packets; they then orbit in an electric potential between inner electrode and housing. The transversely orbiting ions execute harmonic oscillations longitudinally in a parabolic potential well and induce image currents in the electrodes of the housing, which are shaped like a half shell; these image currents can be measured in the form of image current transients and converted into mass spectra.
The patent specification DE 10 2007 024 858 B4 (C. Köster; corresponding to U.S. Pat. No. 7,994,473 B2; GB 2 448 413 B) describes other types of Kingdon ion trap characterized each by the arrangement of several inner electrodes. In this case, also, the inner electrodes and the outer housing electrodes can be precisely shaped in such a way that the longitudinal motion is decoupled from the transverse motion, and a parabolic potential well is created in the longitudinal direction to generate a harmonic oscillation. The embodiments also include those where the analyte ions can oscillate transversely, practically in one plane, in the center plane between at least one pair of inner electrodes; an example is given in FIG. 2. The analyte ions oscillating transversely in this way can then execute harmonic oscillations in the longitudinal direction, and the image current of these oscillations can be measured in order to produce highly resolved mass spectra.
Kingdon ion traps must be operated under ultra-high vacuum if ions are to be stored for a length of time, say a few seconds. During this time the ions must not suffer any collisions with the residual gas because they would then lose kinetic energy and finally hit the inner electrode arrangement. These prolonged storage times are necessary in order to measure the longitudinal oscillation frequencies for high mass resolutions. The measuring times for high resolutions range from a few tenths of a second to a few seconds. The ion species to be analyzed are preferably pulsed into a Kingdon ion trap in as short a time as possible; the (shortest) oscillation period of the lightest ion species in the Kingdon ion trap often determines the time which is available for the introduction of all the other ion species (acceptance time).
The combination of a 3D RF ion trap and a Kingdon ion trap, which is described above as being optimal, is difficult if the 3D RF ion trap is still to be operated as a high-quality mass analyzer, however. The collective ejection of the ions from the 3D RF ion trap requires the RF voltage, which serves as the storage voltage, to be switched off abruptly, preferably within a single RF period. But a 3D RF ion trap which is also operated as a mass analyzer is usually operated in resonance with an RF resonant circuit of high quality and low effective resistance because this is the only way to keep the energy consumption low and achieve extremely high voltages. The quality Q is defined as Q=1/tan δ, where δ is the loss factor. High quality means high resonance sharpness. An RF voltage generated with high-quality resonance in this way cannot be switched off abruptly, however. The resonant circuit simply continues to oscillate when the energy input is switched off; installing a short-circuit switch interferes with the quality of the resonant circuit. The quality of the resonance switching circuit in RF ion traps is around a few hundred, i.e. the amplitude of the RF voltage after the energy input has been switched off decreases over a few hundred RF periods, and thus in around a hundred microseconds.
The collective ejection of ion species of different masses from RF ion traps which are not used as high-quality mass analyzers has been described at various places in the literature.
Document U.S. Pat. No. 7,498,571 B2 (A. A. Makarov et al., granted 2009) describes the ejection from an RF storage device, where the storage unit can be either a 3D RF or a 2D RF ion trap. This is expressly operated solely as an ion storage device, however, and not as a mass analyzer; but it is operated with an RF voltage in resonance via the secondary winding of a transformer, as is usual for mass analyzers. The RF voltage can be switched off abruptly for the ejection by means of a short-circuit switch with a short-circuit resistance (shunt). This circuit configuration is only possible if the maximum RF voltage amounts to only a few hundred volts, but not when a top-quality secondary circuit is required to supply RF voltages of up to several ten kilovolts. In this case any connection with switches or similar has an interfering effect; the switch would also have to be high-voltage-proof, which usually makes it slow. Furthermore, experience shows that the energy content of a circuit with maximum quality cannot be destroyed in one RF period even when a shunt is used. Applicant considers this document to be the closest prior art.
The document U.S. Pat. No. 7,256,397 B2 (E. Kawato and S. Yamaguchi, granted 2007) describes an RF ion trap whose content is pulsed out axially into the flight path of a time-of-flight mass spectrometer. Here too, the RF ion trap is operated only as an ion storage system, so only relatively low RF voltages are required. The special feature here is that, by switching off the RF voltage at a specific phase and switching on the ejection pulse of the time-of-flight spectrometer with a time delay, a mode of operation can be achieved where the kinetic energy of the ions no longer depends on the RF voltage which existed before the pulsed ejection.
The document US 2008/0035842 A1 (M. Sudakov and L. Ding) describes how ions from a linear RF ion trap (2D ion trap) are pulsed axially in a direction radial to the rod system into the flight region of a time-of-flight mass spectrometer (TOF) or into a time-of-flight mass spectrometer with orthogonal acceleration of the ions (OTOF). The RF voltage has the form of a square-wave voltage here, i.e. it is not a sinusoidal voltage, in order to allow rapid switching to a stationary ejection state.